The present invention relates to a fuel injection control system for an automobile engine to calculate a fuel injection quantity from an air induced quantity in cylinders of the engine in dependency on a throttle opening degree and an engine speed.
Generally, in the fuel injection control system of the type described above, a basic injection quantity Tp is first calculated with an induced air quantity and an engine speed as parameters and an actual fuel injection quantity Ti is then calculated by correcting the basic injection quantity Tp with various factors for the correction.
The induced air quantity is measured by an induced air quantity sensor arranged on a directly downstream side of an air cleaner in a L-jetronic system. On the other hand, the induced air quantity is estimated in response to the throttle opening degree (.alpha.) and the engine speed (N) in a so-called ".alpha.-N" system. The ".alpha.-N" system makes simple or compact the engine unit and, hence, is superior from the viewpoint of economics because of fewer problems. In these advantageous view points, the ".alpha.-N" system is widely used for various types of the engine units.
The air quantity induced into the cylinder has a time-lag of first order with a certain time constant. The time-lag of first order occurs according to a lag of changing an intake manifold with air. The induced air quantity estimated in response to the throttle opening degree and the engine speed at a transient state takes a value larger than an actual air quantity in the cylinder and, hence, an air-fuel ratio becomes rich when the throttle valve is rapidly opened at the transient state.
Particularly, in an MPI (multi-point injection) type engine, a calculation timing of the fuel injection quantity supplied into the respective cylinders is set just before the intake stroke, that is an intake valve is about open. So that, at the transient state wherein the induced air quantity is changed during the intake stroke, there occurs a difference between the induced air quantity at the calculation timing of the fuel injection quantity and the air quantity in the cylinder at the completion of the intake stroke. The difference adversely affects air-fuel ratio control characteristics.
In order to obviate such defect, the Japanese Patent Laid-open Publication No. 60-43135 discloses a system wherein an actual air quantity induced into the cylinder is estimated in dependency on the throttle opening degree at the initial stage of the transient state and the engine speed. The fuel injection quantity is changed with the time-lag of first order, so as to reach the fuel injection quantity corresponding to the estimated induced air quantity. Thus, an improvement in the air-fuel ratio control characteristics is attempted.
However, in the described prior art, there is no disclosure of means for estimating the required induced air quantity in dependency on the throttle opening degree and the engine speed.
In another aspect, in a prior application of the same applicant of the present application (Japanese Patent Application No. 63-257645), there is disclosed a system wherein an induced air quantity at this moment is first obtained in dependency on the throttle opening degree and the engine speed. Then the obtained air quantity is corrected by the correction factor depending on the subtracted difference between the obtained air quantity and the preliminarily obtained air quantity. Thus, the intake air quantity approximate to the actual air quantity induced in the cylinder is obtained.
Thus, as shown in FIG. 7, an estimated intake air quantity Map* set at a fuel injection point A of the first cylinder of BTDC.theta.0 (for example, BTDC 80.degree. CA) before the intake stroke an induced air increasing quantity Map at an intake stroke completion point B is primarily estimated in dependency on the difference between an induced air quantity Map(tn) calculated from the throttle opening degree and the engine speed at the point A and the induced air quantity Map(tn-1) in the preceding cycle. A value obtained by adding the induced air quantity Map(tn) to the estimated induced air increasing quantity Map is the estimated induced air quantity Map* at the fuel injection point A. A basic fuel injection quantity Tp is calculated from the estimated induced air quantity Map* and a desired air-fuel ratio A/F as (Tp=Map*/A/F).
However, an acceleration of an engine equipped with more than four cylinders always starts on the intake stroke of a certain one cylinder and, hence, the aforementioned difference between the calculated air quantity and the actually induced quantity is caused in the present intake stroke of the certain cylinder. Therefore, an induced air quantity becomes lean by a quantity corresponding to a portion shown with hatching lines in FIG. 8.
Such a difference will be also caused during a deceleration cycle as a reverse phenomenon.
As a result, the air-fuel ratio control characteristics at the initial stage of the transient state becomes worse and a good response is not achieved. Moreover, the exhaust gas emission at the transient state becomes worse and, hence, the load to the catalyst increases.